In most cases where it is desired to cool glass forming molds, it has been the practice in the past to accomplish this cooling by the use of compressed air. The compressed air, when in turbulent flow contacts the backs of the molds at high velocity that is sufficient, will carry away a considerable amount of heat and effect a controlled heat transfer from the molds. In the glass bottle forming operation, a charge of glass is first fed, usually by gravity, to a parison mold or a blank mold, as they are sometimes termed, where the glass charge is given a preliminary shape known as a "parison". It is not the purpose of the parison mold to extract heat from the parison to any great extent because the parison must be sufficiently plastic so that it may be expanded by compressed air within a blow mold or final shape mold. The blow mold, however, is intended to impart the final shape to the glass container or article and also to remove sufficient heat from the article so that the formed bottle or container may then be placed, bottom down, on a cooling dead plate without deformation and then be processed through the annealing, inspecting and packing systems. This generally describes the cycle of operation of the well-known IS type glass forming machine. Because the residence time of the glass within the parison mold is considerably less than the residence time within the blow mold, a greater amount of heat will need to be removed from the blow mold, although both molds are cooled at the present time.
The problem of heat removal has become particularly acute in glass container manufacture because a limitation on the speeds of production has been caused by the inability of low pressure air to sufficiently cool the forming molds. Low pressure air that is used requires large volume capacity, relatively high velocity and results in a generation of objectionable noise levels in the forming area. Elimination of noise is a primary advantage of the present invention.
More recently, attempts have been made with some success to cool glass forming molds with liquids such as water. An example of such a successful water-cooled mold system is disclosed in U.S. Pat. No. 3,887,350 to Charles W. Jenkins dated June 3, 1975. In this patent there is disclosed the water-cooling of a mold insert holder with a combination asbestos and graphite sleeve interposed between the holder and the mold insert. Each mold unit is made up of three major parts, a mold insert portion which has a cavity therein, a partial heat transfer barrier sleeve and a water-cooled insert holder. These three elements are assembled together as a composite unit. It is important in the cooling of glass forming molds that heat not be extracted too rapidly or in an uncontrolled manner because if the forming surface of the mold is too cold, it can create checks in the finished container or uneven cooling can result in thick areas that are not desirable. With the use of an insulating sleeve, as shown in this patent, a more uniform temperature distribution has been obtained at the molding surface.
Another mold cooling system is described in U.S. Pat. No. 4,009,017 issued Feb. 22, 1977 to Stanley P. Jones. In this particular patent, the mold hanger is disclosed as containing a bed of particulate material such as iron shot with a system for fluidizing this bed of iron shot and in conjunction with this arrangement, a liquid system of effecting a cooling of the iron shot. The fluidized bed is disclosed as being capable of relatively high heat transfer characteristics, with the bed being fairly insulative when not fluidized.
Commonly assigned U.S. Pat. No. 3,887,350, referred to above, that discloses a glass forming mold for controllably removing heat wherein a thermally insulated layer is interposed between the glass forming surface and the coolant supply, discloses several materials for this insulated layer although asbestos fabric is disclosed as being the preferred material. The present invention is considered an improvement over U.S. Pat. No. 3,887,350 in that it provides an improved barrier material and method of construction which may be readily fabricated to provide the desired degree of heat insulated properties in a controllable and reproducible manner.
Applicant has determined that as a means of avoiding variations in thermal conductivity of existing materials that may be used in sheet form and to provide a system that is less sensitive to assembly techniques for making water-cooled mold units in the manner taught by the above-discussed prior art, the use of compacted particulates offers an attractive concept. When considering the selection of particulates for their heat conductivity properties, the form of the particulates as well as their compositions were explored. Several significant factors emerged, and the configuration of the structure for utilizing the particulates as a heat transfer barrier of reproducible character was given consideration.
It was determined that a heat transfer tube would be a configuration that would lend itself to fairly standard reproduction when considering selecting powders and compaction pressure to achieve a specific heat removal from a forming cavity. Compressed powder heat transfer tubes can be made from a wide variety of powdered material. Metals, graphite, sand and various inorganic materials were successfully used. Tubes ranging in thermal conductivities from 0.1 for diatomaceous earth to 38 Btu/ft.hr..degree. F. for graphite will give heat transfer coefficients of from 16 to 325 Btu/ft..sup.2 hr..degree. F.
When dealing with powdered, particulate material, as a medium for use in conjunction with molds with high heat loads, medium to high conductivity systems have been achieved using aluminum or graphite powder, added to stainless steel or nickel powder. These combinations, in varying amounts have given excellent results for thermal conductivities in the range of 0.5 to 8 Btu/ft.hr..degree. F. Low conductivity systems have been achieved using graphite powder, added to diatomaceous earth, for thermal conductivities from 0.1 to 0.7 Btu/ft.hr..degree. F.
The configuration of heat transfer tubes is considered to possess several advantages over a flat plate insulator, in ease of manufacture, and reproducibility. The testing of the powder thermal conductivity can be done on large batches of powder and either the compaction pressure or composition of the powder can be adjusted to effect a precisely desired thermal conductivity. Furthermore, there is no requirement that machining of mold parts will be critical for the mold cooling system using tubes to give preselected results.
The present invention may be further understood by the explanation of the following examples which relate specifically to the techniques and processes used for producing powdered metal heat transfer tubes in a test apparatus. The test apparatus and the cooling system will closely parallel the functional operation of the invention as applied to a glass forming mold or holder. The test apparatus took the form of metallic test block in which a 1/2" diameter, vertical hole or passage was drilled therethrough in the manner shown in FIGS. 3 and 5. A 1/4" diameter tube of stainless steel was positioned coaxially within the passage thus forming an annulus. This annulus was then filled with powdered metal such as -100 mesh, 316LSS powder. 316LSS powder is a stainless steel powder obtained from Glidden; however, it is a product that may be obtained from other sources. This stainless steel powder was compressed in 1cc increments using a die which fit the annulus with a compressing pressure of 30,000 psi.
The block assembly was then installed in an insulated chamber and heated electrically while cooling water flowed through the inner tube. The data presented in Table I, below, was obtained over a period of several hours, as indicated, with the explanation of the asterisks (*) being found at the bottom of the table. This data revealed the heat transfer property of the tube and demonstrates the anticipated performance of the concept of the invention when applied to the use of a plurality of tubes in surrounding relationship to a glass forming mold. The essentially constant value of k.sub.p obtained after the first two readings was: k.sub.p = 1.181, which is taken as the final value.
TABLE I ______________________________________ TYPICAL MEASUREMENT OF HEAT TRANSFER TUBE PERFORMANCE Material: 316L Stainless Steel, -100 mesh, packed in 1cc increments at 30,000 psi. Tube: .25" SS316 tube in a .50" diameter hole. U.sub.o k.sub.p Time ##STR1## ##STR2## ______________________________________ 9:15 71.5 1.400 9:20 71.8 1.405 9:35 63.1 1.186 62.9 1.182 * 63.0 1.183 63.1 1.187 ** 10:40 62.3 1.166 10:45 62.2 1.165 * 10:55 62.5 1.172 10:59 62.9 1.181 ** 11:30 63.1 1.185 11:35 62.6 1.175 * 11:42 63.3 1.191 11:45 63.4 1.195 12:15 62.5 1.172 12:20 62.7 1.176 * 12:30 63.2 1.188 12:34 63.4 1.194 ______________________________________ *thermally shocked by draining water from system for 5 minutes. **thermally shocked by cooling the block to 180.degree. F, then reheating
The overall heat transfer coefficient termed U.sub.o is based on the area of the hole drilled within the mold. The use of U.sub.o may be obtained from the following relationship: ##EQU1## where:
U.sub.o is the overall heat transfer coefficient based on the hole diameter (drilled in the mold), Btu/ft.sup.2 .degree. F. hr.
A.sub.o is the surface area of the hole drilled in the mold, ft.sup.2.
h.sub.i is the water to metal heat transfer coefficient, Btu/ft.sup.2 .degree. F. hr.
A.sub.i is the inside area of the tube (contacted by water), ft.sup.2.
k.sub.t is the thermal conductivity of the metal tube, Btu/ft.sup.2 hr..degree. F./ft.
A.sub.t is the effective heat transfer area of the metal tube, based on the log mean radius, ft.sup.2.
.DELTA.r.sub.t is the metal tube thickness, ft.
k.sub.p is the metal powder thermal conductivity, Btu/ft.sup.2 hr..degree. F./ft.
A.sub.p is the effective heat transfer area of the powder, ft.sup.2.
.DELTA.r.sub.p is the powder thickness, in ft.
In the operation of the test apparatus the quantity k.sub.p was calculated from the following equations: ##EQU2## where:
C.sub.p heat capacity of water Btu/lb.degree. F.
D.sub.b the log mean diameter of the block metal, between the thermocouples and the drilled hole, ft.
D.sub.i inside diameter of the metal water tube, ft.
D.sub.o diameter of the hole drilled in the block, ft.
D.sub.p log mean diameter of the "powder" insulator, ft.
U'.sub.o is the overall heat transfer coefficient between the block at the thermocouple location and the water, based on the diameter of the hole drilled in the test block.
D.sub.t log mean diameter of the metal water tube, ft.
G mass velocity of water in the tube, lb/ft.sup.2 hr.
h.sub.i heat transfer coefficient from the water to the tube, Btuft.sup.2 hr..degree. F.
k.sub.b thermal conductivity of the metal block, Btu/ft. hr..degree. F.
k.sub.p thermal conductivity of the "powder" insulator, Btu/ft.hr..degree. F.
k.sub.t thermal conductivity of the metal tube, Btu/ft.hr..degree. F.
k.sub.w thermal conductivity of the water, Btu/ft.hr..degree. F.
L length of the heat transfer area, ft.
m flow rate of water, lb./hr.
.DELTA.r.sub.b thickness of the metal block between the thermocouples and the hole drilled in the block, ft.
.DELTA.r.sub.p thickness of the "powder" insulator, ft.
.DELTA.r.sub.t thickness of the water tube wall, ft.
T.sub.1 temperature of water flowing into the heat transfer area, .degree. F.
T.sub.2 temperature of water flowing out of the heat transfer area, .degree. F.
T.sub.b average temperature of the block, .degree. F. (measured .DELTA.r.sub.b outside of the drilled hole).
In considering the possibility of using other metal powders, other materials were selected. The test apparatus described above was used wherein the hole or passage again was 0.50 inches in diameter with a coaxially positioned stainless steel tube having a 0.25 inch outside diameter extending therethrough and the formed annulus being filled with pure aluminum powder. It should be noted that the compacting pressure of 14,400 psi is less than that of the previous example. It can be seen that the conductivity started at a very high value and decreased rapidly. It appeared to be unstable, even increasing once after thermal shocking. Pure nickel also exhibited this behavior, giving erratic values. Neither was considered as having the desired qualities for a successful heat transfer tube.
Further, mixtures of various metal particulates as previously stated, have certain predictable results and metals and refractory oxide mixtures also have been tried. Refractory oxide powder and powdered graphite have had limited success. When considering refractory oxides, it should be understood that inorganic powders, such as talc and the previous described diatomaceous earth are possible candidates for use as the particulates to be used in insulating tubes.
TABLE II ______________________________________ MEASUREMENT OF THE PERFORMANCE OF AN UNSTABLE HEAT TRANSFER TUBE Material: -60+150 mesh aluminum, packed in 1cc increments, at 14,500 psi. Tube: .25" OD SS316 tube in a 0.500" hole Reference: 650.degree. F block temperature; 85.degree. F water. Time ##STR3## ##STR4## ______________________________________ 4:10pm 165.0 5.809 4:15 166.9 5.943 4:30 120.5 3.098 4:35 117.0 2.945 ** next day 133.1 3.706 134.7 3.803 * 11:30am 112.9 2.769 11:33 112.4 2.749 ** 12:20 107.9 2.573 12:22 108.4 2.593 * 12:36 97.9 2.212 98.1 2.215 ______________________________________ *thermal shock by draining water for 5 min. **thermal shock by cooling 180.degree. F and reheating.
Other metal powders, as well as mixtures of such powders with graphite were considered. Tests with uncompressed copper, 316SS, nickel and aluminum powders were conducted and found to be difficult to repeat since the degree of compression of the powder was found to have a significant effect. Copper and nickel were tested for several samples to determine reproducibility. Copper was less reproducible, possibly because of partial oxidation of copper at the higher temperatures.
In an effort to study the repeatability factor in selecting powders, the previously described test apparatus was used to conduct a series of tests wherein the cooling tube was formed by the identical procedure and the heat flow characteristics were carefully monitored so as to give comparable results. These results are given in Table III below. Again note the compressing force was 6,000 psi, which is a different pressure than used in the prior examples.
TABLE III ______________________________________ REPEATABILITY TEST Material: -100 mesh. 316L Stainless Steel, compressed in 1cc increments at 6000 psi, 1/4" tube in 1/2" hole. 650.degree. F block temperature; 85.degree. F water temperature Tube: .25" OD SS316 tube in a .50" hole Test ##STR5## ##STR6## ______________________________________ 1 .563 34.8 2 .568 34.6 3 .522 32.1 4 .559 34.1 5 .551 33.5 6 .562 34.5 Average .554 Std. dev. 3.0% ______________________________________
The effect of the pressure used to compact the powders on the thermal conductivities of 316SS and nickel are shown in Table IV.
TABLE IV ______________________________________ EFFECT OF PRESSURE ON THERMAL CONDUCTIVITY Pressure -100 SS 316L AN100 Nickel psi k.sub.p k.sub.p ______________________________________ 10,000 .521 1.03 .531 1.250 .503 .472 1.14 avg. .+-. 13.6% .507 avg. .+-. 5.1% 20,000 .761 1.82 .929 1.818 .846 .845 1.819 avg. .+-. .08% .844 .845 avg. .+-. 7.0% 30,000 1.084 1.75 1.181 2.17 1.200 1.68 1.171 2.12 1.159 avg. .+-. 4.5% 1.93 avg. .+-. 11.8% 40,000 1.377 ______________________________________
From the foregoing, it can be seen that pressure is a significant factor with regard to the thermal conductivity of the cooling system.
Another factor to consider in the selection of the configuration and parameters to follow in designing a water-cooled mold of the invention for a particular shape of mold is the composition of the particulate used as the compacted layer 16. In addition to pure compositions, consideration has been given to various mixtures of powdered material.
The following Table V gives the comparisons of k.sub.p values for various mixtures of Aluminum and Nickel powders in 316L Stainless Steel, compacted at 30,000 psi in 1cc increments in the previously described test apparatus:
TABLE V ______________________________________ Wt % k .rho. Al Ni 316SS Btu/hr.ft..degree. F ______________________________________ 0 0 100 1.159 0 100 0 1.93 100 0 0 3.80 10 0 90 2.30 10 0 90 2.31 25 0 75 3.38-3.68 25 0 75 3.29-3.46 10 30 60 2.53 ______________________________________
The foregoing discussion and explanations are believed to provide a clear understanding of the objects of the present invention.
It, therefore, is an object of this invention to provide a system for cooling glass forming molds which is reproducible to the extent that by using repeatable techniques, the heat transfer characteristics of the molds will be known.
It is a further object of this invention to provide a mold cooling system that is simple in execution and not subject to any complex moving elements that may have unpredictable results.